1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus having a function of detecting a sheet position in a width direction orthogonal to a conveying direction of a sheet.
2. Description of the Related Art
In an image forming apparatus, skewing, a position displacement, and the like may occur when a sheet is fed from a sheet feeding cassette due to a misalignment of a sheet position loaded in the cassette, a slip between a conveying roller and the sheet at the time of conveying the sheet, and the like. As a result, a relative positional displacement between an image transferred onto a sheet and the sheet occurs, and an image may not be formed in a predetermined position of the sheet.
In order to transfer an image to a predetermined position of a sheet, a skew correction mechanism for sheets or a lateral position correction mechanism for correcting the position of a sheet in a direction (width direction) orthogonal to a conveying direction is mounted.
U.S. Pat. No. 8,079,589 discusses an image forming apparatus including a skew correction mechanism for correcting skewing of a sheet with respect to the conveying direction by causing the leading edge of the sheet to contact a registration roller disposed upstream in a sheet conveying direction.
Further, Japanese Patent Application Laid-Open No. 05-124752 discusses an image forming apparatus in which a side edge position of a sheet is detected by a line sensor, such as, for example, a contact image sensor, and a lateral position correction mechanism moves a roller that conveys the sheet in an axial direction thereof so that the side edge position becomes a predetermined position.
In an image forming apparatus including both the above-described skew correction mechanism and the lateral position correction mechanism, a registration roller may be configured to have two functions, a skew correction function and a lateral position correction function. Further, the upstream side or downstream side of the registration roller is provided with a line sensor for detecting the side edge position of a sheet, and the upstream side of the registration roller and the line sensor is provided with a conveying roller for conveying the sheet to the registration roller.
A sheet material is bent by causing the leading edge of the sheet conveyed by a conveying roller to contact the registration roller in the state in which the registration roller stops, such that the skewing of the leading edge of the sheet is corrected. Next, by rotating the registration roller, the side edge position of the sheet is detected by the line sensor in the state in which the sheet is not bent, and the registration roller is shifted in an axial direction thereof according to the detected side edge position to correct the lateral position of the sheet.
Further, Japanese Patent Application Laid-Open No. 2002-248804 discusses a configuration in which a comparator compares an analog output for one line of the line sensor with a threshold value, and the side edge position of the sheet is detected according to the time from when the comparison is started until an output of the comparator is changed.
When the side edge position of the sheet is detected using a light source and the line sensor, outputs from each pixel of the line sensor facing the vicinity of the side edge position of the sheet are changed with an inclination due to a diffused reflection from a sheet surface. Therefore, it is desirable that the threshold value of the comparator is set to be a midpoint between a detection level of a sheet region and a detection level of a region outside the sheet region.
Further, in order for the line sensor to detect the side edge position of the sheet with high precision, there is a need to perform a light quantity adjustment. In the light quantity adjustment, the light quantity is adjusted so as to prevent a maximum value of the outputs from each pixel of the line sensors from being saturated. In addition, a midpoint between an average value of the outputs from each pixel of a region facing the sheet and an average value of the outputs from each pixel of a region outside the sheet region is determined as the threshold value.
However, the sheet being conveyed is nipped by a plurality of roller pairs disposed apart from each other in a width direction, so that the sheet is in a waving state in a width direction. Therefore, a distance between the line sensor and the sheet is changed according to the position of the sheet in the width direction. For this reason, when the distance between the sheet and the line sensor at the side edge position of the sheet is longer than the distance between the sheet and the line sensor at positions other than the side edge position, the side edge position of the sheet may not be detected with high precision.